While recording a video signal, for example, on a magnetic tape or a videodisc, the signal, as is well known, is recorded in the form of a frequency modulated carrier, since then the amplitude variations can be eliminated to the largest extent by an amplitude limitation. In such recorders, because of defective spots in the support layer, foreign matter, or a variety of other influences, blackout intervals appear in the modulated carrier to be picked up, which lead to signal absence in the reproduced image, in the form of more or less long horizontal striae. This phenomenon is called dropout.
For reducing the influence of such imperfections, it is known, from German Auslegeschrift No. 12 02 315, which corresponds to British Pat. No. 931,525, to provide a switching pulse, which is obtained in an amplitude rectifier from the modulated carrier as soon as a dropout appears, and to use this pulse for switching the signal path, for the duration of the dropout, to a substitute signal. Then, through the substitute signal path, the signal of a preceding line for example, is furnished by means of a delay circuit, for the duration of one or several lines. A fixed d.c. voltage corresponding to a halftone, or a voltage obtained by integration of the video signal, may also serve as the substitute signal.
Consequently, such circuit arrangements require a detector for determining a dropout and for producing the switching pulse.
In a prior art circuitry (German Offenlegunsschrift No. 25 25 074 which corresponds to U.S. Pat. No. 3,909,518), aside from supplying it to a low-pass filter for obtaining the desired video signal, the output signal of the FM detector is also supplied to a second low-pass filter having a substantially larger bandwidth than the first low-pass filter. The output signal of this second low-pass filter is supplied to a maximum comparator and to a minimum comparator. Each of these two comparators delivers a pulse as soon as the video signal at the output of the second low-pass filter fails to attain a minimum value or exceeds a maximum value. The output voltages of the two comparators are added and the sum serves as a switching pulse for switching the signal path to the substitute signal path upon the occurrence of a dropout. In this design, the demodulated signal must be supplied to the comparators with a large bandwidth. Also, two comparators are needed to detect the dropouts in both directions. Moreover, the produced switching pulse must be extended by additional extension circuits, in order to obtain a sufficiently long period during which the signal path is switched to the substitute signal path.